The invention relates to a hydraulic control apparatus for a shifting fork for engaging/releasing at least one gear in a manual transmission.
By way of example, DE 101 34 115 A1 has disclosed a hydraulic control apparatus which comprises a double-acting shifting cylinder with two chambers, as well as a piston unit which guides the shifting fork. A control valve for generating a moderated pressure is connected upstream of the chambers. The piston unit and therefore the shifting fork can be displaced by applying the pressure of the control valve to one of the chambers. The volume of the chamber acted on by the pressure of the control valve increases, while the volume of the other chamber decreases and the oil which is present in the latter chamber passes into an oil sump.
One of the problems of the shifting cylinder described above is that the piston unit, during the transition from an at-rest position into a movement, jolts or does not start to move uniformly. This effect is attributable to the transition from static friction to sliding friction between the shifting cylinder and the piston unit. Since the coefficient of static friction is greater than the coefficient of sliding friction, a greater force is required to set the piston unit in motion than to overcome the sliding friction. Consequently, a high pressure initially has to be built up in one of the chambers in order to set the piston unit in motion. When the pressure required to start up the piston unit has been reached, therefore, the piston unit suddenly starts to move. Consequently, starting up the piston unit gently imposes high demands on the hydraulic control apparatus and/or on the control valve connected upstream of the chambers.